The present invention relates to a semiconductor integrated circuit, and more specifically to a semiconductor integrated circuit including an improved structure of a ground line, a ground plane, or a grounded conductor and an internally provided decoupling capacitor.
In the prior art, semiconductor integrated circuits are developed to increase an integration density while paying attention to a required performance and operation. However, the attention to a ground layer is tenuous, and the prior art semiconductor integrated circuits did not have the ground layer.
Referring to FIG. 1, there is shown a partially broken, diagrammatic perspective view of a prior art semiconductor integrated circuit. As shown in FIG. 1, a conducting line 2 is formed on a principal surface of a substrate 9. In this structure, a problem is almost never recognized in connection with electromagnetic noises radiated from a semiconductor device. As a result, at present, electromagnetic noises radiated from various electronic instruments, attributable to semiconductor devices mounted on a printed circuit included in the electronic instrument, have become a problem attracting attention of people. This problem of electromagnetic noises is expected to become more serious with an increase in an operation speed and in the integration density of the semiconductor device.
As mentioned above, since the prior art semiconductor integrated circuit has no ground layer, a line impedance of an internal (ungrounded) conducting line such as a signal line and a power supply line is not stable, with the result that there occurs a noise attributable to waveform disturbance caused by a line reflection.
Furthermore, since the prior art semiconductor integrated circuit has no ground layer, a path of a feedback current of a current flowing in the semiconductor device is indefinite and therefore complicated, with the result that a large loop is constituted of the current flowing in the semiconductor device and its feedback current. This current loop functions as a loop antenna, which causes the electromagnetic radiation.
Moreover, in the prior art semiconductor integrated circuit, since no decoupling capacitor is internally provided in a power supply line, a spurious high frequency power supply current superposed on the power supply line is flowed out through a lead frame to a circuit external to the semiconductor integrated circuit. This is one of major causes of electromagnetic noises radiated from the electronic instrument.
Japanese Patent Application Pre-examination Publication No. JP-A-04-262567 (an English abstract of which is available and the content of the English abstract is also incorporated by reference in its entirety into this application) proposes a semiconductor device having a power supply line and a ground line. Referring to FIG. 2, there is shown a diagrammatic partial sectional view of the semiconductor device proposed in JP-A-04-262567. JP-A-04-262567 describes that, as shown in FIG. 2, a conducting line 2 which is a power supplying line and a ground line 4 are formed in a substrate 9, so that a parasite capacitance 30 formed between the conducting line 2 and the ground line 4, is caused to function as a bypass capacitor. However, the magnitude of the parasite capacitance mentioned in JP-A-04-262567 is too small, and therefore, a satisfactory effect of the bypass capacitor cannot be so expected.
Accordingly, it is an object of the present invention to provide a semiconductor integrated circuit which has overcome the above mentioned problems of the prior art.
Another object of the present invention is to provide a semiconductor integrated circuit capable of minimizing a noise generation from a circuit which is a major noise source in the semiconductor integrated circuit, in order to prevent the spurious electromagnetic noises radiated from the electronic instruments.
The above and other objects of the present invention are achieved in accordance with the present invention by a semiconductor integrated circuit comprising a microstrip line formed of an ungrounded conducting line formed at a substrate and a ground plane conductor formed at the substrate to oppose to the conducting line separately from the conducting line, and a decoupling capacitor provided at the substrate and electrically connected between the conducting line and the ground plane conductor so as to bypass a high frequency current component flowing through the conducting line to the ground plane conductor.
For example, the conducting line is formed on a principal surface of the substrate, and the decoupling capacitor is provided at the principal surface of the substrate. One end of the decoupling capacitor is electrically connected to the conducting line formed on the principal surface of the substrate, and the other end of the decoupling capacitor is electrically connected to the ground plane conductor through a via hole formed in the substrate.
In addition, the decoupling capacitor can be constituted of a chip capacitor mounted on the substrate, or a gate insulator film capacitance which is formed in a MOS transistor formed at the substrate, or alternatively, a PN junction capacitance of a PN junction formed in the substrate.
In a preferred embodiment of the semiconductor integrated circuit, the conducting line is formed of a power supply conductor, and a conductive pad is formed on the principal surface of the substrate for electrically connecting the power supply conductor to an external terminal. In this case, the one end of the decoupling capacitor is electrically connected to the conductive pad on the principal surface of the substrate.
In brief, the semiconductor integrated circuit in accordance with the present invention is characterized in that the decoupling capacitor for bypassing a high frequency current component flowing in the ungrounded conducting line of the microstrip line, to the ground, is provided at the substrate independently of a parasite capacitance. In addition, the ground plane conductor which was not considered in the prior art semiconductor integrated circuit, is positively formed at the substrate to oppose to the conducting line (which can be a power supply line or a signal line) separately from the conducting line. With this arrangement, a feedback path of the current, namely, a loop current area, can be minimized, so that the electromagnetic radiation noises can be effectively suppressed.
Here, if the ground plane conductor is formed to oppose to the ungrounded conducting line so as to constitute the microstrip line, another problem may be encountered in which a power consumption of the semiconductor integrated circuit is increased. In this case, the ground plane conductor is partially formed in a limited region, for example, to oppose to only a critical line (such as a clock line or a bus line), by counterpoising the overall power consumption with the advantage of the ground plane conductor.
Furthermore, if the decoupling capacitor, not the parasite capacitance, is positively connected between the power supply line and the ground plane conductor, a high frequency current component flowing through the power supply line can be effectively bypassed to the ground. In this case, the large the capacitance of the decoupling capacitor is, the characteristic impedance of the line can be reduced, and also, the power consumption can be lowered.
As mentioned above, if the ground plane conductor is formed to oppose to the conducting line separately from the conducting line in the semiconductor integrated circuit, the microstrip line is constituted. The characteristic impedance xe2x80x9cZxe2x80x9d of the microstrip line is stable, and can be expressed by the following equation (1) under the assumption having no edge effect:
Z={2xc3x97(xcexcxc3x97∈)xc2xd}/Cxcfx89xe2x80x83xe2x80x83(1)
where xcexc is permeability of dielectric material
∈ is permittivity of dielectric material
C is capacitance between the conducting line and the ground plane conductor.
If the microstrip line is constituted as mentioned above, the impedance is stabilized, so that waveform disturbance caused by a line reflection can be suppressed, and therefore, a noise attributable to the waveform disturbance can be minimized.
Furthermore, since the ground plane conductor is formed to oppose to the conducting line separately from the conducting line in the semiconductor integrated circuit so as to constitute the microstrip line, a feedback current of a high frequency current component flowing through the conducting line flows through the ground plane conductor directly under the conducting line, in a direction opposite to that of the current component flowing through the conducting line, so that a current loop is constituted. The following equation (2) expresses a remote electric field xe2x80x9cExe2x80x9d in a differential mode radiation caused by the looped current.
E=131.6xc3x971031 16 (f2A I) (1/r) sin xcex8(V/m)xe2x80x83xe2x80x83(2)
where f is frequency (Hz)
A is an area of current loop (m2)
I is current (A)
r is free space distance
As seen from the equation (2), in order to suppress the electromagnetic radiation noises, it is necessary to minimize the area of current loop. Since the ground plane conductor is formed so as to constitute the microstrip line, the area of current loop is minimized, so that the remote electric field strength can be made small. In the case of having no ground, the feedback current of the current flowing through the conducting line takes a complicated path, so that a large current loop is constituted, resulting in a large remote electric field strength.
In the preferred embodiment, if the decoupling capacitor is located on and connected to the pad of the power supply line for electrically connecting the power supply line to an external terminal (such as a lead frame), the spurious high frequency power supply current flowing through the power supply line can be bypassed to the ground without flowing out to a circuit external to the semiconductor integrated circuit. If the spurious high frequency power supply current flows out to the circuit external to the semiconductor integrated circuit, the spurious high frequency power supply current is spread to a complicated path in a printed circuit board, resulting in a cause of the electromagnetic radiation noises. This problem can be prevented in the preferred embodiment of the semiconductor integrated circuit in accordance with the present invention, since the spurious high frequency power supply current flowing through the power supply line is effectively suppressed within the semiconductor integrated circuit.
The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.